As the population ages, cognitive decline and Alzheimer's disease (AD) are becoming increasingly important public health issues. Mild cognitive impairment (MCI) is considered a precursor to AD, and early identification of MCI and effective management of risk factors associated with conversion from MCI to dementia is an important step in managing the public health crisis of AD. Unfortunately, MCI is an unstable diagnostic construct, and key predictors of conversion and reversion are poorly understood. Multiple vascular factors and prevalent cardiovascular disease are known risks for the development of AD, so a plausible predictor of diagnostic fluctuations in MCI is the presence of concomitant micro vascular disease in the brain. This proposal will leverage the existing and robust dataset from the Alzheimer's Disease Neuroimaging Initiative to examine neuroimaging markers of micro vascular and microstructural tissue changes in the white matter with cognitive trajectory among cognitively normal older adults and individuals with MCI. We will (1) assess how stable versus progressive cerebral micro vascular disease and micro structural changes in white matter affect maladaptive cognitive aging over time and (2) assess the mediating effect of baseline systemic vascular disease (using a vascular health index) in the longitudinal relation between micro vascular and micro structural white matter changes and diagnostic conversion and cognitive trajectory. Insights regarding complex relations between systemic vascular disease and micro vascular or microstructural changes in the brain have important public health implications because they will contribute to early identification of a vulnerable population at high risk for cognitive progression. Furthermore, because vascular risk factors can be modified over the life course to reduce the burden of cerebrovascular disease in older adults, this vulnerable population would benefit most from novel strategies to delay or prevent progression, which is particularly valuable in light of the aging of the population and increasing prevalence of AD.